Converting a flat roof to pitched: what's involved

The usual reasons are leaks that keep returning, a covering that is worn out, or a wish to gain headroom or a loft space. Pitch lets water run off by gravity, forgives more in the details, and can be covered in tile or slate that suits the house.

It is a different job from replacing a flat roof like for like. There is no membrane to peel off and relay. The old deck is stripped or left as a ceiling, and a new timber structure rises above it. That means design, a load path to the walls and a structural check. If you are still deciding whether the flat roof is worth saving, start with our guide to flat roof repair or replacement.

A pitched roof pushes outward as well as down, so it needs a proper arrangement of rafters, ceiling joists or trusses and wall plates, and the walls beneath must be able to take the load. Flat roof joists are often sized only to span the room and carry a thin covering, so they are rarely suitable to carry a new pitched roof as they stand.

Two routes are common. A cut roof is framed on site from rafters and ties, which suits odd shapes and tight access. A trussed roof uses factory-made trusses craned or hand-lifted into place, which is quicker where the plan is simple. Each needs a design from a structural engineer or truss supplier, and the walls may need a new wall plate and restraint straps.

Weight matters. Approved Document A treats a change of covering that adds around 15% to the roof load as significant, so a heavy stone or concrete covering in place of a light one calls for a check on what carries it. The Planning Portal suggests having an engineer or surveyor look at it first.

Raising a flat roof into a pitch changes the building's height and outline. Like-for-like work that leaves a building looking the same is not development in planning law, but a new ridge and slope plainly change how it looks. That is why a conversion is the kind of job where you check with Stroud District Council's planning team early, before drawings are finalised.

Extra care applies in a conservation area, in the Cotswolds National Landscape or on a listed building. Stroud District has 41 conservation areas, with Article 4 Directions applying in five, so ask the planning team whether any of them reach roofs on your street. A neighbour's light, the roofline of a terrace and the materials on the street all feature in how a proposal is judged. Photographs of the existing roofline and the houses either side help the conversation.

A new roof structure is structural work, so it needs Building Regulations approval through the Gloucestershire Building Control Partnership, which serves Stroud District, or through a registered building control approver. We explain the consent route and submit the Building Regulations notification where the work needs one.

Insulation is a second strand. Under Approved Document L, a new roof element on an existing dwelling may not exceed a U-value of 0.15 W/(m²·K), so the new pitched roof is built to a higher standard than the old flat one probably met. Ventilation follows BS 5250 guidance: for a cold roof under HR underlay, Marley's reading of that code calls for a 25 mm eaves gap for pitches of 15 degrees or less and 10 mm above that. Pitch also has to suit the covering, and Marley's pitch table lists minimums for each tile, from 15 degrees for a clay pantile upward. Our answer on single-ply PVC on a house extension covers a different option if a full pitch is not wanted.

Every conversion differs, so the figure comes only from the written quote that follows our visit. The main drivers are easy to name, and that quote should cover each of them.

• Structure. Engineer's design, new timbers and wall plates, and any strengthening of the walls.
• Scaffold. A bigger, taller roof needs access all round, and Checkatrade's October 2026 guide puts house re-roof scaffold at about £700 to £1,300.
• Covering. MyBuilder's 2026 guide puts slate re-roofing at £100 to £180 per m² installed, and tile or stone sit at different levels.
• Insulation and ventilation. The new build-up has to meet current standards.
• Details. Verges, valleys, hips, flashings against neighbouring walls, rainwater goods and making good the ceiling below.
• Fees. Building control, any planning application and the engineer.

Your quote shows VAT, which is charged at the standard rate on roofing work.

Begin with the survey. We photograph your existing roof, the walls it sits on and the neighbouring roofline, and note the access for scaffold and materials. From those we can say what the structure will need and which questions to put to the council.

Then comes a written quote covering scaffold, strip, structure, covering, insulation, waste and making good. The finished job carries a 10-year workmanship guarantee, supplied in writing, and our guide to what guarantee a new flat roof should come with shows how to read any guarantee you are offered. See the wider flat roofing page for the options if you decide to stay flat.